1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus using the same.
2. Description of the Related Art
In recent years, with a use of the large number of pixels in an image pickup element and a progress in a technology of digital image processing, a digital camera has substituted a silver salt 35 mm film camera. Moreover, since the number of pixels of a small-size liquid-crystal panel has also increased, it has been made possible to use the small-size liquid-crystal panel as a finder. Therefore, there has been a shift in an interchangeable lens camera from a so-called single lens reflex type to a camera with a new concept in which, a quick-return mirror is eliminated.
In the camera with a new concept, there is no quick-return mirror. Therefore, regarding an image pickup optical system to be used in such camera, designing of the optical system in which, a back focus is short to some extent, and which has a high imaging performance, has become possible.
In the single lens reflect type, a size of a finder image achieved by the image pickup optical system has been dependent on an imaging size (image-circle diameter) of the image pickup optical system. Whereas, in the camera with a new concept, such constraint will cease to exist. Therefore, even if a format size of the image pickup element is small, a large finder image is achieved. Moreover, designing of an image pickup optical system with a small size and high performance has become possible.
On the other hand, as a camera in which, the back focus is short and the format size of the image pickup element is small, a small-size camera (commonly called as a compact camera) in which, the image pickup optical system is fixed to a camera main-body has been used widely. As a zoom lens appropriate for such small-size camera, zoom lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2007-47538 and 2008-304708 are available. Zoom lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2007-47538 and 2008-304708 include in order from an object side five lens units namely, a positive lens unit, a negative lens unit, a positive lens unit, a negative lens unit, and a positive lens unit.
In an arrangement in which, refractive power arrangement (arrangement of refractive power of lens units) is in order of a positive refractive power, a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power, is a one of arrangements called as a positive-lead type. The most basic arrangement as a zoom lens of the positive-lead type is a four-unit zoom lens (zoom lens which includes four lens units in order of a positive lens unit, a negative lens unit, a positive lens unit, and a positive lens unit). The refractive power arrangement in order of a positive refractive power, a negative refractive power, a positive refractive power, a negative refractive power, and a positive refractive power is an arrangement in which, a lens unit having a negative refractive power is added to this four-unit zoom lens, between the third lens unit and the fourth lens unit. By doing so, a zooming ratio is made high at a telephoto end at which, an overall optical length of the zoom lens is prone to increase, while shortening the overall optical length of the zoom lens.
Incidentally, in a zoom lens of an interchangeable lens camera, it is important to achieve a balance of the overall optical length of the zoom lens and a sum total of thickness of lenses. Moreover, there is a restriction on an amount sticking out from a camera installation flange surface. Therefore, in the zoom lens of the replaceable lens camera, a length of some extent is necessary for the back focus. On the other hand, a level of image quality sought in the lens replaceable camera is higher than a level of image quality in a compact camera. Therefore, it is necessary to use an image pickup element having a format size of certain extent in the replaceable lens camera, upon securing even larger number of pixels (However, a pixel size of a fixed image pickup element of recent years being smaller than silver salt particles, even when the size is made smaller than a 135 format or an APS-C, a satisfactory image quality is achieved).